Printing unit having screen printing cylinders and transfer cylinders forming printing nip

ABSTRACT

A printing unit includes two screen-printing cylinders and two transfer cylinders that cooperate with the screen-printing cylinders and which define a printing gap or nip. At least one of the screen-printing cylinders has a screen surface and an interior doctor blade. A support element is provided for handling radially outwardly directed pressure exerted by the doctor blade on the screen surface in one section of the screen surface.

FIELD OF THE INVENTION

The present invention is directed to a printing unit with first andsecond screen-printing cylinders. Transfer cylinders cooperate with thescreen-printing cylinders.

BACKGROUND OF THE INVENTION

EP 07 23 864 B1 discloses a printing unit for a rotary printing press.This printing unit has a first screen-printing cylinder and a printingnip formed by two cylinders, in which a print stock is printed.

A screen-printing cylinder of this prior printing unit forms a printingnip in cooperation with a counter-pressure cylinder, which hascircumferential sections, each with a reduced radius in which sheetgrippers are disposed. On the interior of the screen-printing cylinder,a doctor blade device is provided, and is oriented toward the printingnip. This doctor blade device presses against the screen of thescreen-printing cylinder and pushes ink through it. In order to preventthe screen from being deformed, when a circumference section with areduced radius passes through the printing nip, a mechanism is providedwhich pulls the doctor blade device back from the screen at these times.

This prior art printing unit is only suitable for one-sided printing.The mechanism for retracting the doctor blade device is complex.

JP 11-129599 A discloses a screen-printing press with twoscreen-printing cylinders and a sheet-feeding transfer cylinder.

EP 0 949 069 A1 describes an offset printing press with cooperatingtransfer cylinders, which can be preceded by a screen-printing unit.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a printing unit.

The object of the present invention is attained according to theinvention by the provision of a printing unit with first and secondscreen-printing cylinders. Each screen-printing cylinder is associatedwith its own transfer cylinder. The two transfer cylinders cooperate toform a printing nip.

The advantages that can be achieved with the present invention arecomprised particularly in the fact that it permits first forme printingand second forme printing, in the screen-printing process, with matchingregisters to be executed in a single printing procedure. Thescreen-printing cylinder is distinguished in particular by itsuncomplicated construction.

In the printing unit in accordance with the present invention, at leastone of the two screen-printing cylinders prints a first side of theprint stock indirectly by use of a transfer cylinder. This transfercylinder is the second of the two cylinders that form the printing nipand can therefore simultaneously serve as a counter-pressure cylinderfor printing the second side of the print stock.

Preferably, the first of the two cylinders that form the printing nip isalso a transfer cylinder. This results in an essentially symmetricalconfiguration of the printing unit of the present invention, with thesame printing properties on both sides of the print stock.

In order to also permit an exact, register-matching printing, inrelation to the edge of a sheet-like print stock, preferably at leastone of the two cylinders that form the printing nip is equipped with aholding mechanism for the sheet-like print stock.

To be able to apply ink to the screen-printing cylinders, thesescreen-printing cylinders are each suitably provided with a doctor bladedevice on its interior, which doctor blade device is for use in pressingink through a screen mounted on the screen cylinder. A device forpreventing the doctor blade device from deforming the screen that itpresses against is also advantageously provided in a specificcircumferential section of the screen. This device prevents the screenfrom being excessively deformed when it is pressed against the holdingmechanism or when it is pressed into a channel, which is complementaryto the holding mechanism and which is disposed on the respective othercylinder forming the printing nip. This device thus prevents the screenfrom wearing too rapidly or from being damaged.

A device of this kind can be or can include an adjusting mechanism,which retracts the doctor blade device radially inward when the specificcircumferential section of the screen-printing cylinder passes in frontof the doctor blade device. In accordance with a second feature of thepresent invention, the screen deformation preventing device is a supportelement, which extends axially on the screen-printing cylinder in thevicinity of the specific circumferential section and serves to absorb apressure exerted by the doctor blade in a radially outward direction onthe specific circumferential section.

In order to prevent synchronization errors of the screen-printingcylinder, when the doctor blade device comes into contact with thesupport element and/or leaves contact with it, the support element ispreferably provided with leading and trailing edges for the doctor bladedevice. These leading and trailing edges extend at an angle to ageneratrix of the circumferential surface of the screen cylinder. Sincethe doctor blade device necessarily extends parallel to such ageneratrix, the doctor blade device does not come into contact or leavecontact with the support element over its entire span at one time, butonly at certain points. In addition, braking forces acting on thescreen-printing cylinder therefore remain low and are distributed over afinite section of the circumference of the screen-printing cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention are shown in the drawingsand will be described in detail below.

FIG. 1 is a schematic side elevation view of a printing press with aprinting unit in accordance with the present invention;

FIG. 2 is a schematic side elevation view of the printing unit of themachine depicted in FIG. 1;

FIG. 3 is a schematic side elevation view of a printing unit inaccordance with a second preferred embodiment of the present invention;

FIGS. 4a and 4 b each show a detail of a screen-printing cylinder and atransfer cylinder cooperating with it, in two phases of the rotationalmovement of the cylinders;

FIG. 5 is a side elevation view of a modification of the screen-printingcylinder depicted in FIG. 4a; and

FIG. 6 is a side elevation view of a portion of a screen-printingcylinder and a transfer cylinder according to another preferredembodiment of a printing unit in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1, there may be seen a schematic view of,for example a sheet-fed rotary printing press in which a printing unit4, in accordance with the present invention, is used. The printing presshas a sheet feeder 1 with a sheet stacker 2, from whose top, which isautomatically kept at a constant height, sheets to be printed are fedindividually, or in a continuous stream, by a belt conveyor 3 to theprinting unit 4, which takes the sheets one at a time, prints them, andoutputs them to a chain conveyor 6, which, in the case of multicolorprinting, feeds them to other printing units like the printing unit 4or, as shown in FIG. 1, feeds them directly to an output stack 5.

The sheets pass through the printing unit 4 from top to bottom. A largerscale depiction of the printing unit 4 is provided in FIG. 2 .

Two transport cylinders 7, the upper one of which is shown onlypartially in FIG. 2, are each provided with sheet grippers in arespective section 8 of their circumferences in order to take the edgesof sheets, which are to be printed, from the first belt conveyor 3 in aregister-matching manner. The lower one of the two transport cylinders 7rolls in contact with a first transfer cylinder 9. The first transfercylinder 9 is embodied, for example, as a first rubber blanket cylinder9 and is likewise equipped with a sheet gripper in a section 11 of itscircumference for use in receiving the sheets from the lower transportcylinder 7. The first transfer cylinder or rubber blanket cylinder 9also rolls in contact with a first screen-printing cylinder 12, so thata printing pattern is transferred from the first screen-printingcylinder 12 to it, which printed pattern is printed on a sheet conveyedin a first nip 13 between the lower transport cylinder 7 and the firstrubber blanket cylinder 9. The structure of the first screen-printingcylinder 12 will be discussed in more detail below.

After passing through the first nip 13, a sheet to be printed reaches asecond nip 14 between the first rubber blanket cylinder 9 and a secondtransfer cylinder 16, which is also embodied as a rubber blanketcylinder 16. In this second nip 14, the sheet is printed on its secondside with a pattern that has been transferred to the second transfer orrubber blanket cylinder 16 from a second screen-printing cylinder 17.

The second rubber blanket cylinder 16 does not have a sheet gripperassembly. Instead, a section 18 of its circumference is provided with achannel, which permits the sheet gripper assembly 11 of the first rubberblanket cylinder 9 to pass through the nip 14.

Since the first and second transfer or rubber blanket cylinders 9 and 16and with them, the first and second screen-printing cylinders 12 and 17as well, can rotate in a coupled fashion, it is easy to achieve an exactand durable register matching of the printing on the front and backsides of the sheets by executing a manual or automatic fine adjustmentof the printing formes mounted on the first and second screen-printingcylinders 12 and 17 in relation to each other in the axial direction andin the circumference direction of the cylinders 9; 16; 12; 17.

The various cylinders 7; 9; 16; 12; 17 discussed above are eachsupported at their ends in lateral side frames or mounts 19; 21; 22, aseen in FIG. 2. In the printing unit shown in FIG. 2, these lateral sideframes or mounts 19; 21; 22 are comprised of a central module, whichsupports the transport cylinders 7 and the two rubber blanket cylinders9; 16, as well as two lateral modules, which each support one of thescreen-printing cylinders 12; 17. The end plates of each module areconnected to each other so that they form a rigid frame unit, which canbe removed from the printing unit 4 along with the screen-printingcylinder 12 or 17 that it supports. A module of this kind can bereplaced, for example, by another module, which contains a formecylinder, e.g. for conventional flat offset printing or the like. Thispermits the printing units, such as printing unit 4 to be easily adaptedto a variety of requirements in order to print documents, in whichdifferent printing techniques are used for the front and back sides,with matching registers in a single pass.

FIG. 3 depicts a simplified configuration of the printing unit shown inFIG. 2. In this simplified configuration, elements that correspond tothose of the printing unit from FIG. 2 are provided with the samereference numerals. In the configuration of the printing unit of FIG. 3,the second rubber blanket cylinder 16 is omitted and instead, the secondscreen-printing cylinder 17 forms the second printing nip 14 directlywith the first rubber blanket cylinder 9. The first rubber blanketcylinder 9 consequently performs the function of a counter-pressurecylinder for the second screen-printing cylinder 17.

Examples of suitable configurations of the screen-printing cylinder orcylinders 12 and 17, for use in the printing unit of the presentinvention will be described below in conjunction with FIGS. 4a, 4 b, and5.

FIGS. 4a and 4 b each show a partial section of the firstscreen-printing cylinder 12 in the vicinity of the first nip 13, whichit forms with the first rubber blanket cylinder 9. The structuresdescribed here, however, can also be used in the same way for theconfiguration of the second screen-printing cylinder 17.

The first screen-printing cylinder 12 has a support ring 20 at each ofits axial ends, whose outer circumference has a screen 23 stretched ontoit. Screen 23 may be made of silk or polyamide gauze or bronze wiremesh. On the interior of the screen-printing cylinder 12, a doctor blade24 is provided, whose position in the radial direction is controlled bya curved body, in this instance a curved guide slot 26 located at theends of the screen-printing cylinder 12, and through which a cylindricalguide projection 27 of the doctor blade 24 extends. Outside of thescreen-printing cylinder 12, the guide projection 27 is supported atboth ends so that it can move in the direction of a line 28 connectingthe rotation axes of the two cylinders 9 and 12. FIG. 4a shows thedoctor blade 24 in a position in which the sheet gripper mechanism 29 ofthe first rubber blanket cylinder 9 is passing through the first nip 13between the two cylinders 9 and 12. Opposite from the sheet grippermechanism 29, the screen 23 has a screen section 31 that is indentedradially inward. The guide slot 26 has an arc-shaped guide slot section33, not shown completely in FIG. 4a, which is concentric to thecylindrical outer surface of the screen 23, and an inwardly indentedguide slot section 32, whose curvature corresponds to that of the screensection 31. The curvature of the guide slot section 32 is selected sothat when the guide slot section 32 moves past the cylindrical doctorblade guide projection 27 during the rotation of the firstscreen-printing cylinder 12, the doctor blade 24 is retracted radiallyinward so far that it exerts only a minimal pressure against the screen23, which produces no appreciable deformation of the screen 23 in thescreen section 31, or is retracted so far that it loses all contact withthe screen 23 and consequently exerts no pressure on screen section 31,which could deform this screen section and could damage it during thecourse of operation.

FIG. 4b shows the position of the doctor blade 24 after the guide slotsection 32 has passed the guide projection 27. The arc-shaped section 33of the guide slot 26 keeps the doctor blade 24 pressed against theinside of the screen 23 so that ink 34 disposed against the doctor blade24 is pressed through the open regions of the screen 23 and is thustransferred to the first rubber blanket cylinder 9.

FIG. 5 shows a modification of the first screen-printing cylinder 12depicted in FIGS. 4a and 4 b. Elements that correspond to those of thefirst screen-printing cylinder 12 described above are provided with thesame reference numerals and will not be described again. The structureof the screen-printing cylinder 12 depicted in FIG. 5 is simplifiedthrough the elimination of the guide slot 26. A spring device, which isnot specifically shown in FIG. 5, keeps the doctor blade 24 pressedagainst the screen 23. The doctor blade 24 consequently remains incontact with the screen 23 even when the indented screen section 31passes the doctor blade 24 during the course of the rotational motion ofthe screen-printing cylinder 12. When the indented screen section 31passes in front of the doctor blade 24, the doctor blade 24 is pushedback counter to the force of the spring device toward the axis of thescreen-printing cylinder 12. In order to prevent the doctor blade 24from deforming the screen 23 in the screen section 31, the screen 23 issupported by an externally situated support element 36. The supportelement 36, as seen in FIG. 5 has the shape of a basin or of a troughthat is uniformly curved in cross section, and is, for example, made ofsheet metal or a rigid plastic, and is anchored at its two axial ends tothe ends of the screen-printing cylinder 12. Preferably, the leading andtrailing ends of the screen 23 are also disposed in the screen section31 and are covered by the support element 36. The support element 36 canbe adjusted in the radial direction, as indicated by arrow 37 in orderto set the tension of the screen 23.

During the rotation of the cylinders 9 and 12 of the printing unit,since the support element 36 respectively coincides with the sheetgripper holding mechanism 29 of the rubber blanket cylinder 9 andconsequently no ink can be accepted from the screen-printing cylinder 12in the vicinity of the screen section 31, it is useful for the supportelement 36 to be embodied as a closed plate, which does not permit anyink to pass through the screen section 31 and reach the exterior of thescreen-printing cylinder 12.

FIG. 6 is a detailed depiction of another preferred embodiment of ascreen-printing cylinder 17.

The screen-printing cylinder 17 depicted in FIG. 6 is cylindrical overits entire circumference, and is without an indented section. It istherefore suitable for cooperating with a second cylinder, which has noradially outward-protruding elements such as sheet gripping mechanismsfor a print stock. With reference to the printing unit shown in FIG. 1,the second screen-printing cylinder 17 can have the configuration shownin FIG. 6. Hence in the description that follows, the screen-printingcylinder is identified at 17 and the cylinder that cooperates with it isidentified at 16.

As was the case of the screen-printing cylinder depicted in FIGS. 4a, 4b, and 5, a doctor blade 24 is disposed on the inside of thescreen-printing cylinder 17 shown in FIG. 6, and pushes a paste-like ink34 through the screen 23 stretched on the screen-printing cylinder 17.Doctor blade 24 exerts a radially outward pressure on the screen 23. Inthe cooperating rotation of the screen-printing cylinder 17 and therubber blanket cylinder 16, as long as the screen 23 of thescreen-printing cylinder 17 touches the surface of the rubber blanketcylinder 16, this cylinder 16 supplies a counter-pressure, whichprevents the doctor blade 24 from deforming the screen 23. In order toprevent such a deformation of the screen 23, even in the vicinity of thechannel 38 of the cylinder 16, in which the screen 23 is not in contactwith the rubber blanket cylinder 16, a support element 39 is disposedradially inside the screen 23. The support element 39 extends in thecircumference direction of the screen-printing cylinder 17 over asection 41 of the screen 23, which corresponds to the span of thechannel 38 on the rubber blanket cylinder 16. The support element 39 isconfigured as a closed plate made of metal or rigid plastic, which iscurved in the form of a cylinder segment.

In this configuration depicted in FIG. 6, the support element 39 isattached radially inside the screen 23 and the leading end 42 andtrailing end 43 of the screen 23 overlap each other in the section 41 ofthe screen 23 which is supported by the support element 39. Thus, thesensitive connection between the two ends 42 and 43 of the screen 23,which can be welded for example, is protected from contact with thedoctor blade 24 and is therefore protected from premature wear.

A spring element for moving the doctor blade 24 in the radial directioncould also be provided in this embodiment of a screen-printing cylinder17 depicted in FIG. 6. However, since in this embodiment, the innerradius of the surface of the screen 23 that the doctor blade 24 sweepsacross, is uniform or continuous, the configuration of thescreen-printing cylinder 17 can be further simplified by eliminating theadjustability of the doctor blade 24 so that the fluctuations in theradius are compensated for solely through an elastic deformation of anelastic lip 44 of the doctor blade 24 touching the screen 23.

In a configuration of this kind, in order to facilitate the transitionof the doctor blade 24 from the screen 23, onto the support element 39,and back onto the screen 23 again, and in order to avoid synchronizationerrors of the screen-printing cylinder 17, the support element 39 isprovided with a leading edge 46 and a trailing edge 47, which arebeveled in the circumference direction of the screen-printing cylinder17. In addition, the present invention provides that these edges 46 and47 do not extend exactly parallel to a generatrix of the outer surfaceof the screen-printing cylinder 17 or to the lip 44 of the doctor blade24, but extend at a slight angle to them. For example, a sawtoothed,rafter-shaped, or sinusoidal curve of the edges 46 and 47 isconceivable. Preferably, the edges 46 and 47 each are configured as ahelix with a pitch that is a multiple of the axial length of thescreen-printing cylinder 17. When the doctor blade 24 is running onto oroff these support element edges, such a curvature of the edges 46 and 47prevents braking or acceleration forces acting on the screen-printingcylinder 17 from being exerted only at a certain point in time and at aparticular angular position of the screen-printing cylinder 17. Instead,these forces are distributed over a circumferential section of thescreen-printing cylinder 17 which, depending on the dimensions of thescreen-printing cylinder 17, can be from several millimeters up to a fewcentimeters wide. This force distribution smoothes the torque requiredto drive the screen-printing cylinder 17 and prevents synchronizationerrors.

While preferred embodiments of a printing unit in accordance with thepresent invention have been set forth fully and completely hereinabove,it will be apparent to one of skill in the art that various changes in,for example the specific drives for the various cylinders, the specificsheet gripping mechanisms and the like can be made without departingfrom the true spirit and scope of the present invention which isaccordingly to be limited only by the following claims.

What is claimed is:
 1. A printing unit comprising: a firstscreen-printing cylinder; a first transfer cylinder cooperating withsaid first screen-printing cylinder; a second screen-printing cylinder;and a second transfer cylinder cooperating with said secondscreen-printing cylinder, said first and second transfer cylindersforming a printing nip adapted to print materials passing through saidprinting nip.
 2. The printing unit of claim 1 further including a sheetgripper mechanism on at least one of said first and second transfercylinders.
 3. The printing unit of claim 1 wherein each of saidscreen-printing cylinders includes a screen surface and furtherincluding means for preventing deformation of said screen surface of atleast one of said screen-printing cylinders at a section of said screensurface.
 4. The printing unit of claim 3 wherein each of saidscreen-printing cylinders includes an interiorly positioned doctor bladeand further wherein said screen section deformation preventing meansincludes a guide slot in said screen-printing cylinder, said guide sloteffecting radially inward movement of said doctor blade when said screensurface section is aligned with said doctor blade.
 5. The printing unitof claim 1 wherein said first screen-printing cylinder is a firstremovable module and further wherein said second screen-printingcylinder is a second removable module, said first and second removablemodules providing a modular configuration of the printing unit.